1. (Rarely "lamp-post error") A problem with the discrete equivalent of a boundary condition, often exhibited in programs by iterative loops.From the following problem: "If you build a fence 100 feet long with posts 10 feet apart, how many posts do you need?"(Either 9 or 11 is a better answer than the obvious 10).For example, suppose you have a long list or array of items, and want to process items m through nhow many items are there?The obvious answer is n - m, but that is off by onethe right answer is n - m + 1.The "obvious" formula exhibits a fencepost error.See also zeroth and note that not all off-by-one errors are fencepost errors.The game of Musical Chairs involves a catastrophic off-by-one error where N people try to sit in N - 1 chairs, but it' s not a fencepost error.Fencepost errors come from counting things rather than the spaces between them, or vice versa, or by neglecting to consider whether one should count one or both ends of a row.2. (Rare) An error induced by unexpected regularities in input values, which can (for instance) completely thwart a theoretically efficient binary tree or hash coding implementation.The error here involves the difference between expected and worst case behaviours of an algorithm.[Jargon File](1994-12-01)